Azidothymidine (AZT) is one of the most effective drugs presently available for alleviating some of the symptoms of acquired immunodeficiency syndrome (AIDS) and decreasing its mortality rate. Recently AZT has been recommended and approved for use by HIV positive, asymptomatic patients, including children. Yet its side effects are often dangerous. Although much has been learned about the drug and its activity as an antivirial agent, very little is known of effective serum levels or the relationship (if any) between serum levels and therapeutic index. At the present time only HPLC and RIA methods are available for monitoring AZT serum levels. They are relatively slow, expensive, labor intensive and impractical for routine pharmacokinetic studies. It is the objective of the proposed program to develop and refine a simple, inexpensive, and relatively rapid assay for AZT in human serum, urine and other body fluids by means of a laser light scattering bioassay.